Mailers: Share Code

Prepare

In our settings (lib/bookshelf.rb), there is code block that allows to share the code for all the mailers of our application.
When a mailer includes the Hanami::Mailer module, that block code is yielded within the context of that class.
This is heavily inspired by Ruby Module and its included hook.

Imagine we want to set a default sender for all the mailers.
Instead of specifying it for each mailer, we can use a DRY approach.